1. Field of the Invention
The present invention relates to a road-surface condition estimating device in a vehicle that estimates a road-surface condition with high accuracy on the basis of a relationship between a road-surface friction coefficient and a slip rate of the wheels.
2. Description of the Related Art
In recent years, there have been proposed and put in practical use various control technologies for vehicles, such as traction control, braking-force control, and torque-distribution control technologies. In many of these technologies, the calculation or correction of required control parameters is implemented in accordance with the road surface on which the vehicle is running and the grip condition of the tires. For example, Japanese Unexamined Patent Application Publication No. 6-323171 discloses a technology for setting a correction torque which is to be subtracted from a driving torque on the basis of a slip amount. To describe this technology in more detail, when the vehicle speed of a four-wheel drive vehicle is below a predetermined value of, for example, 20 km/h, a longitudinal acceleration detected by a longitudinal acceleration sensor is subjected to a filtering process. Subsequently, a filtered longitudinal acceleration having undergone a peak-hold process is selected. In contrast, when the vehicle speed is above or equal to the predetermined value, a longitudinal acceleration not having undergone a peak-hold process is selected. Then, the vehicle speed is determined by integrating the selected longitudinal accelerations. A difference between the determined vehicle speed and an average speed of a plurality of speeds included in the rotation speeds of the wheels is regarded as a slip amount, and a correction torque to be subtracted from a driving torque is set on the basis of this slip amount.
In the technology disclosed in Japanese Unexamined Patent Application Publication No. 6-323171, however, the vehicle speed is determined by simply integrating the longitudinal accelerations but is not determined in view of the case where the road on which the vehicle is running is an ascending/descending slope. Therefore, the vehicle speed cannot be determined with high accuracy, which implies that the correction torque also cannot be determined with high accuracy. Specifically, referring to FIG. 3, when the vehicle is running on an ascending/descending slope, a longitudinal acceleration Gx detected by a longitudinal acceleration sensor is affected not only by an acceleration (dV/dt) in the traveling direction of the vehicle but also by gravity.Gx=(dV/dt)+g·sin(θ)  (1)
In this case, g indicates a gravitational acceleration, and θ indicates a slope angle of the road.
A vehicle speed VB obtained by temporally integrating the longitudinal acceleration signal based on the equation (1) is as follows:VB=V0+·(Gx)dt=V0+∫((dV/dt)+g·sin(θ))dt  (2)
In this case, V0 indicates an initial speed at the time when the integration is started.
Consequently, if the vehicle speed VB is estimated based on the detection value Gx detected by the longitudinal acceleration sensor when the vehicle is on an ascending slope, the estimated speed will unfavorably be higher than the actual speed by the amount of the gravity component. Similarly, the vehicle speed VB will be lower than the actual speed when the vehicle is on a descending slope, thus eliminating the ability to perform a proper slip detection.